1. Technical Field
The present invention relates to a liquid developer and an image forming apparatus.
2. Related Art
A developer for developing an electrostatic latent image formed on a latent image carrying member includes a dry toner used in a dry state constituted by a material containing a colorant, such as a pigment, and a binder resin, and a liquid developer (liquid toner) constituted by a toner dispersed in an electrically insulating carrier liquid (insulating liquid).
A method using a dry toner is advantageous owing to the use of a toner in a solid state, but has a concern of adverse influence of powder to a human body and has a problem in contamination due to scattering of the toner and unevenness upon dispersing the toner. A dry toner is liable to suffer aggregation, and the size of the toner particles is difficult to reduce, whereby a toner image having high resolution is difficult to form. In the case where the size of the toner particles is relatively small, the aforementioned problems due to the powder form of the toner conspicuously arise.
In a method using a liquid developer, the toner particles in the liquid developer are effectively prevented from being aggregated, whereby fine toner particles can be used, and a binder resin having a low softening point (softening temperature) can be used. Accordingly, the method using a liquid developer is advantageous in reproducibility of a thin line image, favorable in gradation reproducibility, and excellent in color reproducibility, and is suitable for a high-speed image forming method.
However, an insulating liquid having been used in a liquid developer is constituted mainly by a petroleum hydrocarbon. In the liquid developer, the insulating liquid is attached to the surface of the toner particles upon fixing. The fixing strength has been decreased due to the presence of the insulating liquid attached to the surface of the toner particles in the liquid developer, it has been difficult to obtain sufficient fixing property.
Such an attempt has been made for solving the problems that a naturally derived oil, such as vegetable oil, is used to improve fixing strength through oxidation polymerization reaction of the oil upon fixing (as described, for example, in JP-A-2006-251252. However, the fixing strength is slightly improved with the liquid developer using a naturally derived oil, but the improvement thereof is not satisfactory. Furthermore, offset of the like problems occur upon fixing at a low temperature, which has been practiced owing to energy saving in recent years.